Esters of polyhydric alcohols, also called polyol esters, find a wide range of varying uses in industry, for example as plasticizers or lubricants. The selection of suitable starting materials allows the physical properties, for example boiling point or viscosity, to be controlled, and the chemical properties, such as hydrolysis resistance or stability to oxidative degradation, to be taken into account. Polyol esters can also be tailored to the solution of specific performance problems. Detailed overviews of the use of polyol esters can be found, for example, in Ullmann's
Encyclopedia of Industrial Chemistry, 5th edition, 1985, VCH Verlagsgesellschaft, vol. A1, pages 305-319; 1990, vol. A15, pages 438-440, or in Kirk Othmer, Encyclopedia of Chemical Technology, 3rd edition, John Wiley & Sons, 1978, vol. 1, pages 778-787; 1981, vol. 14, pages 496-498.
The use of polyol esters as lubricants is of great industrial significance, and they are used particularly for those fields of use in which mineral oil-based lubricants only incompletely meet the requirements set. Polyol esters are used especially as turbine engine and instrument oils. Polyol esters for lubricant applications are based frequently on 1,3-propanediol, 1,3-butanediol, 1,4-butanediol, 1,2-hexanediol, 1,6-hexanediol, neopentyl glycol, trimethylolpropane, pentaerythritol, 2,2,4-trimethylpentane-1,3-diol, glycerol or 3(4),8(9)-dihydroxymethyltricyclo[5.2.1.02,6]-decane, also known as TCD alcohol DM, as the alcohol component.
Polyol esters are also used to a considerable degree as plasticizers. Plasticizers find a variety of uses in plastics, coating materials, sealing materials and rubber articles. They interact physically with high-polymeric thermoplastic substances, without reacting chemically, preferably by virtue of their dissolution and swelling capacity. This forms a homogeneous system, the thermoplastic range of which is shifted to lower temperatures compared to the original polymers, one result being that the mechanical properties thereof are optimized, for example deformation capacity, elasticity and strength are increased, and hardness is reduced.
A specific class of polyol esters (they are referred to as G esters for short) contains diols or ether diols as the alcohol component, for example ethylene glycol, diethylene glycol, triethylene glycol, tetraethylene glycol, 1,2-propylene glycol or higher propylene glycols. They can be prepared in different ways. In addition to the reaction of alcohol and acid, optionally in the presence of acidic catalysts, further processes are employed in practice to obtain G esters, including the reaction of diol with acid halide, the transesterification of a carboxylic ester with a diol, and the addition of ethylene oxide onto carboxylic acids (ethoxylation). In industrial manufacture, only the direct reaction of diol and carboxylic acid and the ethoxylation of carboxylic acids have become established as production processes, preference usually being given to the esterification of diol and acid. This is because this process can be performed with no particular complexity in conventional chemical apparatus, and it affords chemically homogeneous products. Compared to this, ethoxylation requires extensive and costly technical equipment.
The direct esterification of alcohols with carboxylic acids is one of the basic operations in organic chemistry. In order to increase the reaction rate, the conversion is typically performed in the presence of catalysts. The use of one of the reactants in excess and/or the removal of the water formed in the course of the reaction ensures that the equilibrium is shifted in accordance with the law of mass action to the side of the reaction product, i.e. of the ester, which means that high yields are achieved.
Comprehensive information regarding the preparation of esters of polyhydric alcohols, also including esters of ethylene glycols and fatty acids, and regarding the properties of selected representatives of these compound classes can be found in Goldsmith, Polyhydric Alcohol Esters of Fatty Acids, Chem. Rev. 33, 257 ff. (1943). For example, esters of diethylene glycol, of triethylene glycol and of polyethylene glycols are prepared at temperatures of 130 to 230° C. over reaction times of 2.5 to 8 hours. Suitable catalysts mentioned for the esterification of polyhydric alcohols are inorganic acids, acidic salts, organic sulfonic acids, acetyl chloride, metals or amphoteric metal oxides. The water of reaction is removed with the aid of an entraining agent, for example toluene or xylene, or by introducing inert gases such as carbon dioxide or nitrogen.
The production and the properties of fatty acid esters of the polyethylene glycols are discussed by Johnson (edit.), Fatty Acids in Industry (1989) Chapter 9, Polyoxyethylene Esters of Fatty Acids, and a series of preparative hints are given. Higher diester concentrations are achieved by the increase in the molar ratio of carboxylic acid to glycol. Suitable measures for removing the water of reaction are azeotropic distillation in the presence of a water-immiscible solvent, heating while passing through an inert gas, or performing the reaction under reduced pressure in the presence of a desiccant. When the addition of catalysts is dispensed with, longer reaction times and higher reaction temperatures are required. Both reaction conditions can be made milder by the use of catalysts. In addition to sulfuric acid, organic acids such as p-toluenesulfonic acid and cation exchangers of the polystyrene type are the preferred catalysts. The use of metal powders, such as tin or iron, is also described. According to the teaching from U.S. Pat. No. 2,628,249, color problems in the case of catalysis with sulfuric acid or sulfonic acid can be alleviated when working in the presence of activated carbon.
Further metallic catalysts used to prepare polyol esters are also alkoxides, carboxylates or chelates of titanium, zirconium or tin, for example according to U.S. Pat. No. 5,324,853 A1. Such metal catalysts can be considered as high-temperature catalysts, since they achieve their full activity only at high esterification temperatures, generally above 180° C. They are frequently added not at the start of the esterification reaction, but after the reaction mixture has already been heated up and has reacted partly with elimination of water. In spite of the relatively high reaction temperatures and relatively long reaction times required compared to the conventional sulfuric acid catalysis, crude esters with a comparatively low color number are obtained in the case of catalysis with such metal compounds. Common esterification catalysts are, for example, tetraisopropyl orthotitanate, tetrabutyl orthotitanate, tetrabutyl zirconate or tin(II) 2-ethylhexanoate. Metal traces in the purified polyol esters can impair the use thereof as plasticizers or lubricants since, for example, the electrical conductivity or the stability to atmospheric oxygen is affected. The prior art proposes a number of measures for converting the esterification catalyst to efficiently removable conversion products.
According to the mode of operation described in DE 10 2009 048 775 A1, the esterification of polyols with aliphatic monocarboxylic acids is conducted with a Lewis acid catalyst in the presence of an adsorbent. In the course of the workup of the crude ester, steam treatment is effected, in the course of which Lewis acid catalyst still present is destroyed. By filtration together with the adsorbent, it is possible to remove the catalyst conversion products in a simple manner. The steam treatment is conducted at temperatures generally of 100 to 250° C. and over a period of 0.5 to 5 hours. During the heating period until the attainment of the working temperature, it is necessary to proceed very gently in order to avoid excessive thermal stress on the crude ester. Especially in the case of preparation of polyol esters based on ether diols, for example triethylene glycol or tetraethylene glycol, the conditions of the steam treatment should be set in a controlled manner, in order to prevent unwanted degradation of the ether chain to by-products. Furthermore, the steam treatment for destruction of the Lewis acid catalyst is time-consuming and impairs the product output achieved per unit reactor volume and time.
It is likewise known that it is possible by addition of water and subsequent treatment with alkaline reagents to convert the Lewis acid catalyst to conversion products having good removability. According to the mode of operation disclosed in DE 30 12 203, the crude ester is admixed with 5% to 50% by weight of water, based on the amount of crude ester, and then heated. The heat treatment with water forms well-crystallized conversion products of the Lewis acid catalyst. The water treatment is then followed by the treatment with alkali.
WO 2007/095262 A2 concerns the esterification of 1,3-propanediol which is obtained from renewable raw materials with fatty acids containing 8 to 40 carbon atoms in the molecule, in the presence of metallic catalysts. After the reaction has ended, the catalyst can be removed by treatment with water.
DE 10 2009 060 865 A1 discloses a process for preparing polyol esters which is conducted in the presence of tin compounds. The crude ester obtained is aftertreated by addition of water. After the aqueous phase has been removed, the polyol ester is optionally treated with a sorbent.
DE 40 02 949 A1 discloses a process for workup of a crude esterification mixture which is obtained by continuous esterification in the presence of metallic catalysts. After the unconverted alcohol has been distilled off, the mixture is cooled, activated carbon is added and the residual alcohol is stripped off with steam or nitrogen in the presence of the activated carbon.
According to the procedure from U.S. Pat. No. 5,324,853 A1, the crude esterification mixture is admixed with an aqueous sodium carbonate solution and optionally with activated carbon. This procedure hydrolyzes the metal compounds to insoluble solids, which can be filtered off before the further workup of the crude ester compound.
In the case of workup of a crude ester mixture which is obtained by reaction of polybasic carboxylic acids with monoalcohols in the presence of Lewis acid catalysts, for example of titanium- or tin-containing catalysts too, the prior art recommends a water treatment for catalyst removal. According to U.S. Pat. No. 5,434,294, the crude ester is treated with an aqueous alkaline solution at temperatures between 80 and 150° C. and then filtered through an adsorbent. In the workup method described in DE 1 945 359, the crude ester is first treated with alkali and the free alcohol is removed by a steam distillation. Subsequently, the product is cooled down to a temperature below the boiling point of the water and then admixed with at least 0.5% by weight of water, based on the product to be worked up. By means of this water treatment, precipitates of the catalyst conversion products having good filterability are obtained.
In order to ensure sufficient removal of the Lewis acid catalyst in the form of catalyst conversion products after the esterification reaction has ended, the prior art teaches a steam treatment in the presence of an adsorbent or a treatment with water at temperatures below the boiling point of water at the particular pressure in conjunction with a treatment of basic compounds. Since, however, steam treatment is time-consuming and the conditions have to be controlled, and a treatment with bases, for example with sodium hydroxide or sodium carbonate, additionally introduces salts which have to be removed again in the course of the crude ester workup, there is therefore a need for a process for aftertreatment of polyol esters which is less time-consuming but simultaneously provides the desired polyol ester in adequate quality, such that there is reliable compliance with the required specification values such as residual acid number, water content, hydroxyl number and residual metal content and the polyol esters can have maximum versatility of use.